The present invention relates to a sheet feeding mechanism for a printing apparatus which feeds or discharges print sheets with two confronting roller units, each of which is composed of rollers and a shaft.
A sheet feeding mechanism for a printing apparatus which feeds or discharges print sheets with two confronting roller units generally has a structure wherein the group of rollers constituting one of the two confronting roller units is not biased by an independent loading device. Specifically, as shown in FIG. 1 (which is taken from Japanese Patent Unexamined Publication No. 197339/1986), bias rollers 31 forming a second roller unit 30 arranged so as to confront drive rollers 29 forming a first roller unit 28 are biased by no independent loading device. Even in cases where the group of rollers constituting one of these two confronting roller units is biased by an independent loading device, the rollers have not been driven thereby. That is, as shown in FIG. 2 (taken from Japanese Patent Unexamined Publication No. 285841/1987), each of bias rollers 35 forming a second roller unit 34 arranged so as to confront drive rollers 33 forming a first roller unit 32 is biased by an independent loading device, but they are not driven thereby.
The above-described sheet feeding mechanisms entail the problem of not providing a feeding force to one of the surfaces of the print sheet if the rollers in one of the pair of rollers are idle. Even if both rollers of the pair of rollers are driven, since neither roller is biased by an independent loading device, the pressure contact force applied to the print sheet is not uniform. When a copy sheet is used as a print sheet, because the two surfaces of the sheet have dissimilar properties, dislocation tends to occur between the front and back surfaces of the sheet when the sheet is fed or discharged, or the sheet feeding force is not stably applied at the roller unit section, thus causing such problems as slanted sheet feeding and out-of-pitch printing.
The invention further relates to a print sheet feed mechanism suitable for use in serial printers.
Because serial printers usually print characters, patterns, and the like on a print sheet while the print sheet is wrapped around the surface of a platen, they are provided with a sheet bias plate on the bottom side of a print head moving path and a paper bale on the upper side thereof. The upper and lower regions interposing the print head moving path are held by these members to prevent dislocation of the sheet.
With this arrangement, nothing can be printed on the head and tail end regions of a sheet in areas defined by the distance between the print head and the paper bale and the distance between the print head and the sheet bias plate, thereby entailing inconveniences when printing slips, labels, and the like.